Both sexes of Siricidae are easily distinguished from all Symphyta (and probably all Hymenoptera) by the collar-like pronotum, and the cornus (horn) present on tergum 10 in females and on sternum 9 in males.
General. Body length 7–38 mm, slender and mostly covered with long, more or less entangled setae. Males and females of the same species often differ considerably in color pattern and adults of most species may show great variation in size.
Head. In frontal view, head markedly constricted below the eye (Fig. A3.4). Malar space with well defined horizontal antennal groove between eye and mandible, sharply outlined ventrally. In posterior view, foramen magnum widely separated from mouth opening by occiput (Fig. C1.1). Mouthparts. Labrum small, finger-like and hidden under clypeus. Mandible with three teeth. Maxillary palp with 1 article; labial with 2 or 3 articles. Antenna. Scape with ventral surface flattened and concave, fitting into antennal groove when retracted (Fig. C1.2); pedicel wider than long and about 0.25 times as long as scape; flagellum with 4–30+ flagellomeres, flagellomere 1 as long as, longer, or shorter than following flagellomere (Fig. A3.11); flagellomeres each with sensory oval impression (often quite sharply outlined especially in female) on part or most of ventral surface (males in some genera, without sensory oval impression on apical flagellomeres) (Fig. A3.25).
Thorax. Pronotum long medially, collar-like with anterior margin slightly concave, and with acute anterolateral corners (Fig. C1.3). Propleura widely touching medially. Mesonotum with median lobe usually without notauli (except in Xeris, notauli slightly outlined in anterior third and far in front of scutellum) and each lateral lobe transversely divided by a wide, deep oblique furrow (possibly the precursor of the transscutal fissure) extending from scutellum posteromedially toward base of fore wing (Fig. C1.4). Tegula very small and mostly hidden under pronotal angle (the “tegula” of authors probably refers to the humeral plate). Mesoscutellum with a small, very narrow and sharply outlined posteromedian appendage. Legs. Tibial spur number: 1 (protibia), 1 (mesotibia), and 1 or 2 (metatibia). Tarsal pad (pulvillus) present on tarsomeres 1-4 and integrated within ventral surface of tarsomeres (slightly extended posteriorly in fresh specimens) (Figs. A3.27 & A3.28), without microtrichae, either smooth or papillate with scattered sensilla, almost as long as ventral length of tarsomeres 3 and 4, 0.4–0.8 times length of tarsomere 2, and 0.01–0.2 times length of tarsomere 1. Protibia with one row of spatula-like setae along posterodorsal margin. Pro- and mesotibiae with dorsal and ventral surfaces curved, commonly appressed to body. Pro- and mesofemora clearly longer than metafemur, their ventral or dorsoventral surface rasp-like, with numerous transverse ridges (Fig. C1.5). Female with tarsomere 1 0.7–1.0 times length of corresponding tibia. (Fig. C1.6). Male with hind leg greatly modified: tibia and tarsomeres 1–3 either thicker (Fig. C1.7) or compressed laterally and leaf-like (much less so on fore and middle legs) (Fig. C1.8); metatibia in lateral view with dorsal edge sinuate or sharply constricted in basal 0.3 and ventral edge sinuate in basal 0.3. Wings. Fore wing cell 3R1 enlarged toward apex (especially apical to vein 3r–m) and cell apex far from wing margin; fore wing veins R1 and Rs2 faint ending with a short petiole (Fig. C1.9); stigma very narrow; cell 1M much narrower (about 0.5 as wide) than cell 2M (Fig. C1.9); vein 2A with basal portion clearly outlined; vein 3A present or absent (Fig. C1.9). Hind wing with only one set of hamuli along edge, either only apical to vein R1 or apical and basal to vein R1 (Figs. B1.11 & B1.12); anal cell absent, or present and with apical petiole almost reaching wing edge (Fig. A3.29).
Abdomen. Tergum 1 divided medially (Fig. C1.10). Terga 2–8 with pit like sculpticells (surface similar to velvet) over most of median area (except in X. matsumurae) (Fig. C1.11). Female with tergum 8 in dorsal view with disc markedly extended posteriorly (Fig. A3.1); tergum 9 in lateral view greatly lengthened (about 0.3–0.5 times abdomen length) (Fig. A3.1) with sharp longitudinal furrows basolaterally (Fig. C1.12) and median basin dorsally (Fig. A3.12); median basin anteriorly outlined laterally by two sets of short furrows and extending posteriorly to base of tergum 10. Tergum 10 separated at least laterally from tergum 9 by transverse furrow and extended posteriorly as a wide or narrow horn (cornus) ending in a rod-like apex (Figs. A3.1 & A3.12); cercus present or absent anterior to anus, and usually very small when present (Figs. B1.31 & B1.32). Ovipositor. Sword-like, with both lance and lancet subdivided into annuli (Figs. A3.16 & A3.17). Lancet with 12-50 annuli, ventrally smooth (without teeth) but apical 3 or 4 annuli each with a large tooth laterally (Fig. A3.17); annuli before teeth annuli laterally each with apical pit, but pit present on as few as 3–5 apical annuli each with a pit (Figs. A3.16 & A3.17); if pits absent on basal annuli, the annuli either outlined or not. Male with sternum 8 deeply cleft medially and sternum 9 extended as a triangular horn ending with a rod-like apex (Fig. C1.13).
Viitasaari (1984) and Viitasaari and Midtgaard (1989) introduced sawfly taxonomists to pits on the ovipositor lancet. The character was first used by Kjellander (1945) to distinguish S. juvencus from S. noctilio. The ovipositor pits turned out to be crucial in deciphering the species of Sirex in the New World. Undoubtedly this character will be important in the study of Euroasiatic species of this genus and other genera.
The following keys to families of Hymenoptera include useful features to identify the family Siricidae: Ross (1937), Benson (1938), Smith (1988), Goulet (1992) and Mason (1993).
Based on species listed by Taeger et al. (2010) and the species recognized here, there are 122 extant species known worldwide. These are grouped into ten genera classified traditionally in two subfamilies. Ninety-seven species are recorded from the Old World mostly in Eurasia, and 28 native species in the New World are known from Guatemala and the Dominican Republic north to the tree line in North America. Except for the introduced Sirex noctilio, Urocerus gigas and Tremex fuscicornis in South America, South Africa, Australia, and New Zealand, and two native species in equatorial Africa, there are no extant native species in the southern hemisphere. Siricidae are not known from oceanic islands.
In the New World, Siricidae are represented by seven extant genera (including one introduced into southeastern United States) and 33 extant species (including five introduced species). One species, Urocerus patagonicus Fidalgo and Smith – a Paleocene fossil from Patagonia (Argentina) – is the only native species recorded from the southern hemisphere (Fidalgo and Smith 1987). Twenty-eight native species are distributed as follows: Sirex – 13 species recorded north of Guatemala and the Dominican Republic, Sirotremex – one species restricted to Mexico, Teredon – one species restricted to Cuba, Tremex – one species in temperate regions of Canada, United States and northernmost Mexico, Urocerus – five species from Mexico northward, and Xeris – seven species from Mexico and northward. We have seen fewer than 25 specimens from Mexico representing five species, three of which are new. No doubt numerous species await discovery in the conifer zone of the Mexican highlands.